1. Field of the Invention
The present invention relates to a wiring board used for a high-frequency package for holding high-frequency devices such as semiconductor devices and passive devices that operate in high-frequency regions, used for a circuit substrate mounting such a package, or used for a circuit substrate directly mounting various devices on the surface thereof. More specifically, the invention relates to a wiring board used being connected to a waveguide to efficiently transmit signals between a signal transmission line and the waveguide.
2. Description of the Prior Art
The trend toward sophisticated information technology in modern society is accompanied by the development in the field of wireless and personalized data transmission as represented by cellular phones. In such circumstances, semiconductor devices have been developed that operate in millimeter wave (30 to 300 GHz) regions to enable the transmission of data at higher speeds and in larger quantities. Following the progress in such a modern technology related to high-frequency semiconductor devices, a variety of applied systems have also been proposed using electromagnetic millimeter waves, such as a radar between cars, wireless LAN, etc. There have been proposed, for example, a radar using millimeter waves (see Electronics Society Convention, Japanese Electronic Data Communication Academy, SC-7-6, 1995), a cordless camera system (see Electronics Society Convention, Japanese Electronic Data Communication Academy, C-137, 1995), and high-speed wireless LAN (see Electronics Society Convention, Japanese Electronic Data Communication Academy, C-139, 1995).
The progress of the application of millimeter waves is also accompanied by the development of devices for realizing the applied use thereof. In particular, it is a serious problem concerned to every kind of electronic component how to decrease the size and cost yet maintaining required transmission characteristics.
In such devices, one serious problem is how to simply connect the circuit substrate or the package holding the high-frequency device to an external electric circuit using a small structure. For example, one significant problem is how to connect the external electric circuit forming a waveguide having the smallest transmission loss to the circuit substrate or the package mounting the high-frequency device.
The high-frequency package has heretofore been connected to the waveguide formed in the external electric circuit by, for example, a method by which a signal transmission line formed in a high-frequency package is first converted into a coaxial line by using a connector and then connected to a waveguide, a method by which the waveguide is first connected to a microstrip line in the external electric circuit, and the microstrip line is connected to a signal transmission line formed in the high-frequency package, and by the like method.
Recently, furthermore, there has also been proposed a method by which the high-frequency package is directly connected to the waveguide of the external electric circuit (see Electronics Society Convention, Japanese Electronic Data Communication Academy, SC-7-5, 1995). According to this proposal, quartz is buried in a portion of a closure member forming a cavity in which the device is air-tightly sealed, and the electromagnetic waves from the waveguide are introduced into the cavity through the portion where the quartz is buried, in order to connect the waveguide to a waveguide-microstrip line converter substrate installed in the cavity.
According to the method by which the waveguide of the external electric circuit is connected to the package through another transmission line such as a connector or a microstrip line as described above, however, the connection structure itself becomes complex, and it becomes necessary to maintain a region for forming the connector or the transmission line, causing a problem in that the connection structure itself becomes bulky. Besides, the transmission loss may increase through the line or the connector.
On the other hand, the method which directly introduces the electromagnetic waves from the waveguide into the interior of the cavity of the package is effective in decreasing the size of the connection structure, but requires the use of a material having a small dielectric constant and a small dielectric loss tangent in order to decrease the loss of electromagnetic waves that pass through the cavity-forming member such as the closure and, hence, requires burying a material having a small dielectric constant and a small loss, such as quartz as taught in the above-mentioned literature. However, such burying not only spoils the reliability of the air-tight sealing but also is not quite suited for the mass production. It can also be considered to use a material having a small dielectric constant and a small loss as a material for forming the cavity. However, the material that constitutes the package requires various characteristics such as mechanical strength, air-tight sealing, metalizing property and like properties in addition to the electric properties. At present, however, no such a material is found that satisfies all of such properties and that can be cheaply produced.
Japanese Unexamined Patent Publication (Kokai) No. 112209/1999 and W096/27913 propose technologies which are capable of air-tightly sealing the device and are capable of connecting signals between the transmission line and the waveguide. According to these technologies, the signals of the microstrip line are connected to the waveguide through an opening formed in the grounded layer and through the dielectric layer, and the waveguide is joined by adjusting the thickness of the dielectric layer under the opening to meet the frequency of the transmission signals and by using the dielectric layer to work as a resonator. Therefore, the thickness of the dielectric layer seriously affects and, accordingly, characteristics vary to a large extent.
Other structures for connecting the transmission line to the waveguide have also been disclosed in DE 4,208,058 and in U.S. Pat. No. 5,793,263, according to which the signal of a microstrip line are connected to the waveguide through an opening formed in the grounded layer and through a dipole antenna formed on the surface of a dielectric. According to these constructions, the waveguide is coupled by adjusting the length of the dipole antenna to meet the transmission frequency. Being affected by the thickness of the dielectric layer under the opening and by the length of the dipole antenna, therefore, it is difficult to strictly control the length of the antenna at the time of manufacturing and, hence, characteristics vary to a larger extent.